The last stage is simply a power MOSFET transistor
that turns the solenoid valve on and off. This circuit is simplicity itself:
the unregulated 12 volt (or whatever) supply is connected to the coil of the solenoid
valve, which in turn is connected to the transistor drain. The transistor
gate is connected the comparator output. The value of R501 is not at
all critical; it is simply there to keep the Gate/Source resistance at a reasonably
low level and to provide a light load for the comparator. The diode across
the coil is standard practice; it prevents the coil's collapsing magnetic field
from generating a big voltage spike that damages and deranges other circuitry.
If you have never used a MOSFET transistor
before, here's a word of caution: Leave the conductive foam it comes with
in place until you are ready to solder the transistor in place. Better yet,
wind a piece of fine wire around the three leads of the transistor, shorting them
together before you remove the foam. Then leave the wire in place, pushed
up against the transistor body, until the MOSFET is fully soldered in place.
(But remember to remove the wire when you are done soldering!) MOSFETs are
wonderful devices, but they really do get blown by static
electricity. Keeping the pins shorted until the transistor is safely soldered
into the circuit will prevent this from happening.